Cool ants forget distance but not direction

Summary

Solitary foraging insects, such as ants, maintain an estimate of the direction and distance to their starting location as they move away from it, in a process known as path integration. This estimate, commonly known as the “home vector,” is updated continuously as the ant moves 1 ,  2 ,  3 ,  4 and is reset as soon as it enters its nest, 5 yet ants prevented from returning to their nest can still use their home vector when released several hours later. 6 ,7 This conjunction of fast update and long persistence of the home vector memory does not directly map to existing accounts of short-, mid-, and long-term memory; 2 ,8 ,  9 ,  10 ,  11 ,  12 hence, the substrate of this memory remains unknown. Chill-coma anesthesia 13 ,  14 ,  15 has previously been shown to affect associative memory retention in fruit flies 14 ,16 and honeybees. 9 ,17 ,18 We investigate the nature of path integration memory by anesthetizing ants after they have accumulated home vector information and testing if the memory persists on recovery. We show that after anesthesia the memory of the distance ants have traveled is degraded, but the memory of the direction is retained. We also show that this is consistent with models of path integration that maintain the memory in a redundant Cartesian coordinate system and with the hypothesis that chill-coma produces a proportional reduction of the memory, rather than a subtractive reduction or increase of noise. The observed effect is not compatible with a memory based on recurrent circuit activity and points toward an activity-dependent molecular process as the basis of path integration memory.